The present invention relates to an index-type color cathode ray tube, in which a phosphor screen, which has a plurality of regions, is dividedly scanned with electron beams that are emitted from a plurality of electron guns.
In general, color cathode ray tubes that are practically used in a wide variety of fields are of the shadow-mask type. In these cathode ray tubes, electron beams incident on a phosphor screen, which is composed of dot- or stripe-shaped phosphor layers that radiate in three different colors, blue, green, and red, are sorted by means of a shadow mask that is opposed to the screen. In the cathode ray tubes of this type, however, the electron beam transmission of the shadow mask is as low as about 20%, so that the radiation efficiency of the phosphor, compared to the energy of all electron beams emitted from the electron guns, is too low to ensure a high luminance of the tubes. Further, the shadow mask is heated to be deformed by the electron beams that run against it, so that the color purity is lowered.
Conventionally, index-type color cathode ray tubes are known as ones that are free from the aforesaid drawbacks of the shadow-mask type. In the index-type color cathode ray tubes, a phosphor screen is provided with phosphor layers for generating index signals, as well as stripe-shaped phosphor layers of three colors for image display. Scanning positions for electron beams are detected for color change by using light sensing units to detect index signal lights obtained from the index signal phosphor layers by electron beam scanning.
These index-type color cathode ray tubes can dispense with a shadow mask and enjoy a high luminance. In the cathode ray tubes of this type, however, the spot diameters of the electron beams with which the phosphor screen are scanned are expected to be not larger than the stripe width of the phosphor layers of each color throughout the phosphor screen area.
The larger the size of a color cathode ray tube, in general, the longer the distance from each electron gun to the phosphor screen is, and the higher the electrooptical magnification of each electron gun is. In the case of a large-sized tube, in particular, therefore, it is difficult to reduce the spot diameters of the electron beams throughout the phosphor screen area.
In order to detect the scanning positions for electron beams continually, moreover, the phosphor screen must be scanned with electron beams that have energy such that index signal lights with a given intensity can be obtained from the index signal phosphor layers. Thus, the brightness of the black level of a picture is settled depending on the lowest level of the electron beam energy. The larger the size of the cathode ray tube, on the other hand, the longer the distance from the phosphor screen to each light sensing unit is. Correspondingly, the index signal lights that reach the light sensing units are feebler, so that the detection of the lights is more difficult. If the energy of the electron beams with which the phosphor screen is scanned is enhanced, the black level of the picture becomes brighter, resulting in a whitened picture of low quality.
As means for solving these problems, an index-type color cathode ray tube is described in Jpn. Pat. Appln. KOKOKU Publication No. 4-53067. In this cathode ray tube, an integral phosphor screen has a plurality of regions, and these regions are dividedly scanned with electron beams that are emitted from a plurality of electron guns. Images generated individually on the screen regions are joined together into one synthetic image to be displayed. Even though this index-type cathode ray tube is large-sized, the distance from the phosphor screen to each light sensing unit can be shortened. Accordingly, the black level of the picture can be prevented from being lowered, and the resulting tube can enjoy a high luminance, high contrast, and large size.
In order to improve the scanning and color change characteristics, some index-type cathode ray tubes are designed so that index signal phosphor layers of two types are formed on the phosphor screen. These phosphor layers include first and second index signal phosphor layers. Each first index signal phosphor layer is formed over a black stripe situated between each two of stripe-shaped phosphor layers of three colors, with a metal backing of aluminum interposed between the layers. Each second index signal phosphor layer is formed extending at a given angle to the color phosphor layers with the aluminum backing between the layers.
Described in Jpn. Pat. Appln. KOKAI Publication No. 57-65651 is photoelectric converter means for general index signal lights. According to this converter means, the index signal lights are subjected to wavelength conversion by means of transparent flat plates (light collecting plates) doped with a phosphor, and are guided to solid light sensing units.
In order to discriminate the index signal lights of two types having different radiation wavelengths, according to known detecting means, a filter is disposed in front of (or on the phosphor-screen side of) each light collecting plate so that only lights of a given wavelength can be guided to the collecting plate, whereby the index signal lights are detected.
In discriminating the index signal lights with different radiation wavelengths by means of filters, however, long-wavelength index signal lights can be identified relatively easily by using a filter that has a predetermined transmission characteristic. Although short-wavelength index signal lights can be also discriminated by means of a filter with a predetermined transmission, it is difficult to manufacture the filter of this type, actually. This filter on the short-wavelength side, if manufacturable, is more expensive than the one on the long-wavelength side. Actually, a substantial difference in transmission cannot be secured between long- and short-wavelength regions of the short-wavelength filter. If the transmission of the long-wavelength region is lowered in order to shade the long-wavelength index signal lights without restraint, therefore, that of the short-wavelength region lowers correspondingly, so that the sensitivity for the detection of the index signal lights inevitably worsens. If the transmission of the short-wavelength region is increased, on the other hand, that of the long-wavelength region also increases, so that the long-wavelength index signal lights are introduced. Thus, it is difficult to change the electron beams.
According to the index-type cathode ray tube in which the integral phosphor screen has a plurality of regions which are dividedly scanned, moreover, an additional component is needed to detect the index signal light for each region, so that the number of necessary components, and therefore, manufacturing costs increase.
In the index-type cathode ray tube constructed in this manner, the filters of the two types are used to discriminate the index signal lights of the two types. In this case, it is difficult to avoid the introduction of the long-wavelength index signal lights so that only the short-wavelength index signal lights can be discriminated, and satisfactory images cannot be obtained. Also, there are problems including the necessity of use of a plurality of expensive filters, high costs, etc.